1. Field of the Invention
The present invention relates to the field of fabrication of microstructures, and, more particularly, to a tool for conditioning the surface of a polishing pad in a system for chemical mechanical polishing of substrates.
2. Description of the Related Art
In microstructures such as integrated circuits, a large number of elements, e.g., transistors, capacitors and resistors, are fabricated on a single substrate by depositing semi-conductive, conductive and insulating material layers and patterning those layers by photolithography and etch techniques. The individual circuit elements are electrically connected by means of metal lines. In the formation of these metal lines, a so-called inter-layer dielectric is deposited and vias and trenches are thereafter formed in this dielectric layer. The vias and trenches are then filled with a metal, e.g., copper, to provide electrical contact between the circuit elements. In modem integrated circuits, a plurality of such metallization layers comprising metal lines must be stacked on top of each other to maintain the required functionality. The repeated patterning of material layers, however, creates a non-planar surface topography, which may deteriorate subsequent patterning processes, especially for such microstructures including features with minimum dimensions in the submicron range, as is the case for sophisticated integrated circuits.
It has turned out to be necessary to planarize the surface of the substrate between the formation of subsequent layers. A planar surface of the substrate is desirable for various reasons, one of them being the limited optical depth of the focus in photolithography which is used to pattern the material layers of microstructures. Chemical mechanical polishing is an appropriate and widely used process to achieve global planarization of a substrate.
FIG. 1 schematically shows a schematic sketch of a conventional system 100 for chemical mechanical polishing. The system 100 comprises a platen 101 on which a polishing pad 102 is mounted. Frequently, polishing pads are formed of a cellular microstructure polymer material having numerous voids, such as polyurethane. A polishing head 130 comprises a body 104 and a substrate holder 105 for receiving and holding a substrate 103. The polishing head 130 is coupled to a drive assembly 106. The device 100 further comprises a slurry supply 112 and a pad conditioner 131. The pad conditioner 131 comprises a conditioning head 107 and a conditioning pad 108 attached to the conditioning head 107. The conditioning head 107 is coupled to a drive assembly 109.
In operation, the platen 101 rotates. The slurry supply 112 supplies slurry to a surface of the polishing pad 102 where it is dispensed by centrifugal forces. The slurry comprises a chemical compound reacting with the material or materials on the surface of the substrate 103. The reaction product is removed by abrasives contained in the slurry and/or the polishing pad 102. The polishing head 130, and thus the substrate 103, is rotated by the drive assembly 106 in order to substantially compensate for the effects of different angular velocities of parts of the polishing pad 102 at different radii. In advanced systems 100, the rotating polishing head 130 is additionally moved across the polishing pad 102 to further optimize the relative motion between the substrate 103 and the polishing pad 102 and to maximize pad utilization. The drive assembly 109 rotates the conditioning head 107 and thus the conditioning pad 108 attached to it. The conditioning pad 108 may comprise an abrasive component like, e.g., diamonds embedded in a matrix. Thus, the surface of the polishing pad 102 is abraded and densified slurry, as well as particles that have been polished away from the surface of the substrate, are removed from voids in the porous polishing pad 102. This process is denoted as conditioning.
Without conditioning, densified slurry and particles abraded from the substrate 103 would clog pores in the polishing pad 102. Thus, the polishing pad 102 would lose its absorbency such that most of the slurry would flow off the polishing pad 102 too quickly. Due to this degradation of the polishing pad 102, the removal rate in the polishing process would steadily decrease.
Conditioning may be performed after polishing one or more substrates 103. This, however, leads to significant variations of the removal rate due to the difference between the reworked surface of a freshly conditioned polishing pad 102 compared to the exhausted surface present immediately before the conditioning. Alternatively, the pad conditioner 131 is continuously in contact with the polishing pad 102 while the substrate 103 is polished. Thus, a more uniform rate of removal of substrate material is achieved.
Various designs of chemical mechanical polishing devices are known in the art. For example, the rotating platen 101 may be replaced with a continuous belt kept in tension by rollers moving at high speed, or slurry may be injected through the polishing pad 102 in order to deliver slurry directly to the interface between the polishing pad 102 and the substrate 103.
One problem with conventional systems for chemical mechanical polishing is that conditioning pads are consumables, which typically have lifetimes of less than 2,000 substrates. Thus, conditioning pads are expensive consumables, the price of which significantly contributes to the cost of operating a chemical mechanical polishing device.
Another problem with conventional systems for chemical mechanical polishing is that conditioning pads comprising diamonds tend to lose single diamonds, which then may cause serious scratches on the surface of the polished substrate. Depending on the type of polishing system and the control strategy thereof, a large number of substrates can be affected until the problem is either detected and removed by pad changes, or the diamond is removed by pad conditioning. This can result in high costs for scratched substrates.
In view of the above-mentioned problems, a need exists for a system for chemical mechanical polishing which comprises an improved pad conditioner.